According to the known prior art, classic ophthalmic instruments for examination of the eye use conventional white light sources for illumination, so as to generate an image of the eye's interior that is as natural as possible for the observer. In order to enable examinations in specific spectral ranges, suitable spectral filters are inserted in the beam path following the white light source.
These conventional white light sources, such as e.g. halogen lamps, have the disadvantage that, from an energetic and economic point of view, the light is generated only with a relatively low efficiency of 8-12%. Moreover, a considerable part of the spectrum is located outside the visible range, and the UV and IR components have to be filtered out in order to prevent the illumination damaging the eye.
This disadvantage is even more pronounced where only very narrow spectral ranges, e.g. from the UV range, are used to carry out fluorescence examinations. Accordingly, this conventional illumination requires very complex technical equipment for the mechanically movable optical filters and for cooling the system.
A further disadvantage of conventional white light sources is their switch-on and switch-off behavior, which is characterized, on the one hand, by relatively long switching times (in the range of >100 ms) and, on the other hand, by variation of the spectral composition of the light during the switch-on phase. Moreover, halogen lamps have a relatively long warm-up phase.
In the slit lamps known from the prior art, which use halogen lamps, the light of the halogen lamp is parallelized by a condenser lens and then illuminates a slit whose width is adjustable. The light passing through the slit is then imaged sharply by an optical system into the anterior chamber of the eye to be examined. Light scattered back by the eye is imaged onto a camera by second detection optics and/or enables visual observation of the eye. In order to be able to vary the angle between the illumination and detection beam paths, the illumination beam path is bent in front of the eye by a prism. This deflecting prism is located approximately in the pupil plane in front of the eye. Since all illumination beams have to transmit through the prism exit surface, said deflecting prism limits the étndue of the source of illumination. In this respect, it is important for the light passing through the slit in the source of illumination to be as homogeneous as possible, because said homogeneity is transmitted as far as the focal plane of the slit lamp due to the imaging into the eye.
Since the homogeneous slit illumination is achieved in the prior art by a condenser lens arranged preceding the halogen lamp, the slit is located in the pupil plane of the halogen lamp's spiral-wound filament, so that the homogeneity in the slit plane thus corresponds to the homogeneity of intensity in the angular spectrum of the halogen lamp's spiral-wound filament.
Due to the relatively long switching times, there is a need, particularly in the case of short exposure times, for an additional, quick shutter which puts the light of a “burnt-in” lamp to its actual use. This is disadvantageous, in particular, in the case of moving objects under examination, such as the eye, because very short exposure times in the ms range are needed here in order to exclude influences of motion when documenting the eye.
Document EP 1,114,608 B1 describes a known embodiment of an ophthalmic irradiation system using in a subcomponent of the total system an illumination on the basis of LEDs. It is set forth in the dependent claims that the device substantially serves to emit certain quantities of red, green, and blue light so as to generate substantially white light. The individual light regulation serves to maintain the respective shade when the protective filter is swiveled in or swiveled out, respectively. Thus, in a special alternative embodiment, the document EP 1,114,608 B1 describes an illumination system on the basis of LEDs, which serves to maintain neutrality in color in combination with an optical protective filter.
Document EP 1,602,323 A1 describes the use of a white LED as the source of illumination in a classic slit lamp. In contrast to the already described classic slit lamp illumination, the homogeneity in the slit plane corresponds to the homogeneity of intensity in the angular spectrum of the LED chip surface here. However, since there is a clear difference between the optical properties of the light emission of a spiral-wound filament and an LED chip surface, this also has negative effects on the achievable homogeneity. Due to the curved shape of the incandescent wire, a spiral-wound filament emits an approximately spherical wave with a homogeneous intensity in the angular spectrum. In contrast thereto, an LED chip functioning as a planar emitter emits, with good approximation, a Lambert angular spectrum. This means that the light intensity decreases with the cosine to the LED chip surface normal, causing systematic trimming in the slit plane. Said trimming depends on the aperture of the condenser lens, and an aperture of NA=1 corresponds to 100% trimming. Such trimming cannot be avoided completely in case of a “Koehler” illumination, but can only be reduced by limiting the aperture of the condenser lens, which on the other hand strongly reduces the energy efficiency of the source of illumination, however. A particular advantage of this apparatus—as compared to slit lamps on the basis of halogen lights—is the light's high consistency in color at different intensities. Thus, EP 1,602,323 A1 describes a classic slit lamp using as a source of radiation a white LED or red, green, and blue LEDs, respectively, to generate white light.
Document U.S. Pat. No. 5,997,141 A describes a system which uses arrays of LEDs for illumination of the eye. Document U.S. Pat. No. 4,699,482 A describes an illumination device which uses LEDs in combination with light-conducting fibers for illumination of the eye.
All these documents relating to spatially distributed emitters have the disadvantage—in the absence of special devices for homogenization—that the intensity in the field of illumination is not sufficiently homogeneous and does not suffice for a sensitive diagnosis.